Activism consists of efforts to promote, impede, direct or intervene in social, political, economic or environmental reform with the desire to make changes in society toward a perceived common good. Forms of activism range from mandate building in a community (including writing letters to newspapers), petitioning elected officials, running or contributing to a political campaign, preferential patronage (or boycott) of businesses, and demonstrative forms of activism like rallies, street marches, strikes, sit-ins, or hunger strikes.

Activism may be performed on a day-to-day basis in a wide variety of ways, including through the creation of art (artivism), computer hacking (hacktivism), or simply in how one chooses to spend their money (economic activism). For example, the refusal to buy clothes or other merchandise from a company as a protest against the exploitation of workers by that company could be considered an expression of activism. However, the term commonly refers to a form of collective action, in which numerous individuals coordinate an act of protest together. Collective action that is purposeful, organized, and sustained over a period of time becomes known as a social movement.

Historically, activists have used literature, including pamphlets, tracts, and books to disseminate or propagate their messages and attempt to persuade their readers of the justice of their cause. Research has now begun to explore how contemporary activist groups use social media to facilitate civic engagement and collective action combining politics with technology. Left-wing and right-wing online activists often use different tactics. Hashtag activism and offline protest are more common on the left. Working strategically with partisan media, migrating to alternative platforms, and manipulation of mainstream media are more common on the right (in the United States). In addition, the perception of increased left-wing activism in science and academia may decrease conservative trust in science and motivate some forms of conservative activism, including on college campuses. Some scholars have also shown how the influence of very wealthy Americans is a form of activism.

Separating activism and terrorism can be difficult and has been described as a 'fine line'.

The Online Etymology Dictionary records the English words "activism" and "activist" as in use in the political sense from the year 1920 or 1915 respectively. The history of the word activism traces back to earlier understandings of collective behavior and social action. As late as 1969 activism was defined as "the policy or practice of doing things with decision and energy", without regard to a political signification, whereas social action was defined as "organized action taken by a group to improve social conditions", without regard to normative status. Following the surge of "new social movements" in the United States during the 1960s, activism came to be understood as a rational and legitimate democratic form of protest or appeal.

However, the history of the existence of revolt through organized or unified protest in recorded history dates back to the slave revolts of the 1st century BC(E) in the Roman Empire, where under the leadership of former gladiator Spartacus 6,000 slaves rebelled and were crucified from Capua to Rome in what became known as the Third Servile War.

In English history, the Peasants' Revolt erupted in response to the imposition of a poll tax, and has been paralleled by other rebellions and revolutions in Hungary, Russia, and more recently, for example, Hong Kong. In 1930 under the leadership of Mahatma Gandhi thousands of protesting Indians participated in the Salt March, as a protest against the oppressive taxes of their government, resulting in the imprisonment of 60,000 people and eventually independence of their nation. In nations throughout Asia, Africa and South America, the prominence of activism organized by social movements and especially under the leadership of civil activists or social revolutionaries has pushed for increasing national self-reliance or, in some parts of the developing world, collectivist communist or socialist organization and affiliation. Activism has had major impacts on Western societies as well, particularly over the past century through social movements such as the Labour movement, the women's rights movement, and the civil rights movement.

Activism has often been thought to address either human rights or environmental concerns, but libertarian and religious right activism are also important types. Human rights and environmental issues have historically been treated separately both within international law and as activist movements; prior to the 21st century, most human rights movements did not explicitly treat environmental issues, and likewise, human rights concerns were not typically integrated into early environmental activism. In the 21st century, the intersection between human rights and environmentalism has become increasingly important, leading to criticism of the mainstream environmentalist movement and the development of the environmental justice and climate justice movements.